Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, an image forming apparatus including a transfer bias output device that outputs a superimposed bias as a transfer bias.
Description of the Related Art
There are known image forming apparatuses equipped with a transfer bias output device that outputs a superimposed bias as a transfer bias in which an alternating current bias and a direct current bias are superimposed, to transfer a toner image onto a recording medium. In the image forming apparatuses of this kind, toner images formed on photosensitive drums through the electrophotographic process are transferred onto a belt-type intermediate transfer member (hereinafter, intermediate transfer belt) and then onto a recording medium at a secondary transfer nip at which the intermediate transfer belt and a secondary transfer roller meet and press against each other.
In this configuration, when using a recording medium having a coarse surface such as Japanese paper and embossed paper, a pattern of light and dark according to surface conditions of the recording medium appears easily in an output image. That is, toner does not transfer well to such embossed surfaces, in particular, recessed portions of the surface. Thus, an image density at the recessed portions is lower than the image density at projecting portions or smooth portions. This inadequate transfer of the toner appears as a pattern of light and dark patches in the resulting output image.
In view of the above, in one approach, a secondary bias composed of a superimposed bias including an alternating current (AC) bias and a direct current (DC) bias is applied to the secondary transfer roller. Using the superimposed bias as a secondary transfer bias enhances transfer of toner to the recessed portions of the surface of the recording medium, thereby preventing the pattern of light and dark patches.
However, without proper control of a peak-to-peak voltage of an AC component of the secondary bias in accordance with transfer conditions that affect transfer of toner such as temperature, humidity, a thickness of the recording medium, a size (depth) of the recessed portions of the recording medium surface, and an amount of toner adhered to the surface of the photosensitive drum per unit area, toner is not transferred well to the recessed portions of the recording medium, resulting in inadequate image density at the recessed portions of the recording medium and hence producing the pattern of light and dark patches.
Furthermore, in a low-temperature, low-humidity environment, a desirable image density is difficult to obtain in the recessed portions of the recording medium in a configuration in which an AC bias is either under constant voltage control or constant current control so as to achieve a target output value for a peak-to-peak voltage of an AC component, and the target output value is changed depending on the transfer conditions such as temperature while supplying a DC component under constant current control or constant voltage control. When a transfer peak value of the secondary transfer bias is too high in a low-temperature, low-humidity environment, electric discharge occurs in the recessed portions of the recording medium in the secondary transfer nip, causing reverse charging of toner particles. Such reverse charging causes toner voids or missing of toner in an image at the recessed portions on the surface of the recording medium, which appears as white dots in an output image.
In view of the above, there is demand for an image forming apparatus capable of preventing an undesirable pattern of light and dark associated with surface conditions of a recording medium.